1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the semiconductor device, and more specifically to a hetero-bipolar transistor and a method of fabricating the hetero-bipolar transistor.
2. Description of the Related Art
In these years, in order to improve the performance of Si devices, an epitaxial growth technology using in-situ doping has been adopted, in which a dopant source material is supplied onto an Si substrate together with an Si source material. With this technology, the thickness and dopant profile in the formed film can be controlled by in-situ doping in epitaxial growth with a high degree of accuracy during an epitaxial growth, and, therefore, it is possible to obtain high performance devices as compared to an ion implantation process technology. Improvement of the performance has also been realized by using quantum effects by epitaxially growing a material having a different band gap, such as SiGe or SiGeC, onto an Si substrate.
Among epitaxial growth methods capable of providing such high performance devices, a chemical vapor deposition (CVD) method has widely been used, in which a source material is supplied in vapor phase and growth is carried out by using a chemical reaction on a substrate surface or in a vapor phase.
Generally, in the case where an epitaxial growth method is used in a process of forming integrated circuits, crystal growth is required on a substrate in which an isolation region and the like have already been formed.
Hereinbelow, conventional epitaxial growth steps on a patterned substrate will be described with reference to FIGS. 9a-9c. FIGS. 9a-9c are sectional views showing steps of growing an epitaxial crystal layer on a substrate according to a conventional fabrication method.
First, in a step shown in FIG. 9a, a shallow trench isolation 104 where a silicon oxide (SiO2) film is buried and a deep trench isolation 105 where undoped polysilicon is buried are formed on a substrate 101 having an active region made of Si crystal, with the active region 102 surrounded by the shallow trench isolation and the deep trench isolation. In this structure, a surface of the substrate 101 consists of an Si surface where the active region 102 is exposed and an SiO2 surface where the shallow trench device 104 made of SiO2 is exposed.
There are generally two kinds of epitaxial growth methods to be performed on a patterned substrate 101 shown in FIG. 9a, which are a selective growth method and a non-selective growth method. Hereinbelow, these two methods will be described with reference to FIGS. 9b and 9c. 
First, the selective growth method will be described. FIG. 9b is a sectional view showing a step of forming an epitaxial layer 106 on the substrate 101 shown in FIG. 9a by a selective growth method. The selective growth method is a method for forming a film only on the active region 102 but not forming the film on the shallow trench isolation 104 in the substrate 101.
Although it has been reported that an epitaxial layer 106 can be selectively formed by optimizing the kind of material gas and crystal growth conditions, such a selective crystal growth is practically difficult since conditions capable of forming the epitaxial layer 106 are difficult to establish and maintain. Therefore, the method for selectively growing an epitaxial growth layer is not suitable for mass production.
Next, the non-selective growth method will be described. FIG. 9c is a sectional view showing a step of forming an epitaxial layer 106 and a polycrystalline layer 107 on the substrate 101 shown in FIG. 9a by a non-selective growth method. The non-selective growth method is a method for simultaneously forming an epitaxial layer 106 on the active region 102 and a polycrystalline layer 107 on the shallow trench isolation 104 in the substrate 101. With the non-selective growth method, the epitaxial layer 106 can be formed relatively easily, and therefore the non-selective growth method is suitable for mass production.
However, in the conventional non-selective growth method, there has arisen a problem that the control of layer thickness of an Si epitaxial layer is technically difficult. It is considered that layer thickness variation of this Si epitaxial layer is due to temperature variations at its growth surface during the layer formation, which will be described below.
Ways to heat a substrate in a CVD method include a cold wall type and a hot wall type.
FIGS. 10a and 10b are views showing ways to heat a substrate in a CVD method, in which FIG. 10a is a view schematically showing a cold wall type, and FIG. 10b is a view schematically showing a hot wall type.
As shown in FIG. 10b, in the hot wall type, a peripheral wall of a reaction chamber 201 is heated by means of a heater 202 or the like, and then a substrate (wafer) 203 placed within the reaction chamber 201 is heated by radiation heat from the peripheral wall. However, in this method, since the peripheral wall of the reaction chamber 201 is heated to a high temperature, a film is also formed on this peripheral wall and comes off when deposited to a certain extent, thereby causing the formation of particles. For this reason, in fabricating high-performance devices, the cold wall type is generally used.
As shown in FIG. 10a, in the cold wall type, the substrate 203 placed within the reaction chamber 201 is heated by means of a heat source 202 that is present within the reaction chamber 201. Therefore, the peripheral wall of the reaction chamber 201 is not heated to a high temperature, thereby preventing a film from being deposited on and from coming off the peripheral wall. It should be noted that, in the method shown in FIG. 10a, the heater 202 is provided as the heat source within the reaction chamber 201, and the substrate 203 is heated by means of this heater. In addition to this method, there are a method in which the substrate 203 within the reaction chamber 201 is heated by electromagnetic induction using a high frequency coil provided around the reaction chamber 201 and a method in which the substrate 203 within the reaction chamber 201 is irradiated with and heated by infrared ray coming from the outside through a window provided in the peripheral wall of the reaction chamber 201. In these cases, the substrate 203 receives energy from the outside of the reaction chamber 201 in the form other than heat and generates. That is, the substrate 203 serves as a heat source.
By the way, when an epitaxial growth is performed on an Si substrate by a CVD method using this cold wall type, the epitaxial growth is performed while the substrate is heated by means of a heat source within a reaction chamber. During the epitaxial growth, the temperature of a growth surface of the substrate is determined as a temperature at which an equilibrium between the amounts of heat supplied from the heat source and emitted from the substrate is maintained. It has been found out that the amount of heat emitted from the substrate depends largely on the material of the substrate surface and the constitution of the substrate, and problems as will be described below have arisen at a surface of the conventional substrate.
Specifically, in the non-selective growth step shown in FIG. 9c, a crystal growth is performed on the active region 102 in the surface of the substrate 101 and on the shallow trench isolation 104. When the substrate 101 is uniformly heated from the outside during this crystal growth, the actual surface temperature of the growth surface of the substrate 101 becomes non-uniform because Si crystal which forms the active region 102 and silicon oxide buried in the shallow trench isolation region 104 have different heat emissivities each other. This non-uniform distribution of the actual surface temperature in the growth surface has made it difficult to achieve eveness of the Si epitaxial layer 106.
The Si epitaxial layer 106 and the polycrystalline polysilicon layer 107 are formed respectively on the active region 102 and the shallow trench isolation 104. A constant amount of heat is given to the polycrystalline polysilicon layer 107 from the heat source through the substrate 101 during the growth of the Si epitaxial layer 106. On the other hand, the heat emitting conditions at a growth surface of the polycrystalline polysilicon layer 107 change as a growing film thickness of the polycrystalline polysilicon layer 107 changes. That is, while the amount of heat absorbed by the polycrystalline polysilicon layer 107 is constant, the amount of heat emitted from the polycrystalline polysilicon layer 107 changes. As a result, the temperature of the growth surface of the polycrystalline polysilicon layer 107 changes.
It is extremely difficult to measure with a high degree of accuracy the surface temperature of the growth surface of the polycrystalline polysilicon layer 107 while the growth proceeds. Accordingly, it is also difficult to change the amount of heat to be supplied from the heat source according to the surface temperature change during the growth of the polycrystalline polysilicon layer 107. Consequently, it has been considered that a surface temperature change of the growth surface is difficult to suppress.
For the reason stated above, there has arisen a problem that the layer thickness of an Si epitaxial layer is difficult to control in the non-selective growth.
This problem arises because the temperature of the growth surface of the substrate changes as the amount of heat emitted from the growth surface changes. Therefore, such a problem does not arise in the hot wall type where the temperature of a growth surface of the substrate is kept constant by emitting heat from a peripheral wall of the reaction chamber. That is, this problem is inherent to the cold wall type.